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| Precise Offset Compensation Ohm Measurements and Validation of DMMs |
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| Dr. Frank:
Ohm Measurement Problem I came across a very uncommon error on ohm measurements, when I checked a brand new 34465A against a 3458A, using my old precision ‘reference’ resistors. Setting on both DMMs was: 4W Ohm for values <= 10k, Offset Compensation for values <= 100k, and NPLC 100. So the highest accuracy of < 1ppm could be expected. For the 100k resistor, there was a gross reading difference of about 15ppm between both instruments. This is no ppm nit-picking, and no volt-nuttery any more, that is a severe measurement problem, obviously. In contrast to that, a 10.0000k Vishay Bulk Metal Foil resistor, which had been accurately measured to 1ppm independently by Third Party, gives identical readings on both DMMs to 1ppm. This resistor serves as a baseline calibration standard for the 3458A, which conducts by its AUTOCAL function all necessary internal precision ratio transfers, calibrating all ranges from 10.00 Ohm to 10MOhm, to about 2ppm uncertainty. As the 34465A was brand new, I concluded, that the 3458A has a failure in its Ohm measurement circuitry, maybe a defect of one of the FETs. This would also discredit the complete AUTOCAL functionality for Ohm, and for the current ranges. At that point, my confidence in the 3458A was completely shattered. From a metrological point of view, that’s the classical ‘man-with-two-clocks’ problem: As the absolute value of the 100k resistor had not been independently measured, it is not possible to make a decision about, which of both instrument is failing. Even worse: As I before had relied on only one precision instrument, i.e. the 3458A with its AUTOCAL function, I had absolutely been ‘blind’ for any possible failures. As a conclusion, an independent verification of the functionality and calibration of any instrument is always needed. |
| Dr. Frank:
Part 2: Resistors These resistors I have salvaged from old HP instruments. These are precision wire wound type, from 1969, <= 0.01%, T.C. 1...5 ppm/K. I used them as shunts and as resistance standards for the last 25 years, also to calibrate my DMMs, like the 34401A. I had determined their absolute values several times with 3 different 3458As, in 1990, 1996, and since 2009 with my own 3458A, every time using that same setting. They all showed a decent drift over the years. The same error occurs on other resistors, like the 5450A (37ppm) and a decade resistor box (18ppm). The 5450A allows an independent 100k calibration from the 10k Vishay resistor, by using a precise 10:1 ratio transfer. Description is here: https://www.eevblog.com/forum/blog/eevblog-544-fluke-5450a-resistance-calibrator-teardown/msg823545/#msg823545 So in the end, it has been decided, that the 3458A displays an erroneous value. And maybe, that I also did not detect that error, for the last 25 years. |
| Dr. Frank:
Part 3: Root Cause I started to measure deeply inside the guts of the 3458A ohm circuitry, especially the timing and found … a totally different explanation of this effect. The 3458A has no defect, and its AUTOCAL is working correctly. When I measured another PWW 100k resistor, both instruments showed exactly the same value. Therefore, this error obviously depends on the DUT. At first I found out, that the timing of the 34465A and the 3458A in OCOMP mode is different on the identical settings. The 3458A always makes NPLC 10 measurements which were averaged to give higher NPLC numbers. So a NPLC 100 ohm measurement consists of 10 x 400ms wide On/Off cycles. The 34465A only makes measurements of 2s on and 2s off. Therefore, this has something to do with charging effects, obviously. Using NPLC 10 lets the 34465A also display wrong values, like the 3458A. |
| Dr. Frank:
Part 4: DMMs with OCOMP So each instrument having an OFFSET COMPENSATION function may be affected, see table. I went through all the user manuals but did not find any clear explanation about this failure, unlike 4W and OCOMP functions, which were extensively explained in measurement tutorials. Also in the specifications, there is no parameter characterizing this problem. The 34420 and the 3458A only had very hidden, and not very obvious hints about that problem. They propose either to switch off OCOMP, or to experimentally introduce a delay. Both measures actually helped, but w/o OCOMP, big errors from thermal voltages can be observed; especially on ranges, where the test voltage is 100mV or 1V only, i.e. on the 10 Ohm.. 100k ranges. Normally all instruments have delays on the order of several 10ms only, the 3458A increases that to 50ms during 10k calibration. The FLUKE 8508A has a very long 5 seconds delay, which makes this instrument extremely slow, but FLUKE probably knew about that problem, and also limited their offset compensation to max. 20k range. The 7 ½ digit Keithley 7510 directly adds 10ppm error for OCOMP on the 100k range, for no obvious reason. Using an appropriate delay of about 5 sec for 100k, and 1 sec for 10k ranges, all the measurements now were correct, and especially the verified 100k value was in accordance to 3458A and 34465A in each case. |
| Dr. Frank:
Part 5: Explanation of the effect Anyhow, to find an explanation for this error, I checked the impedance of these resistors for parasitic capacitance and inductance. I measured the impedance over frequency, and there is a remarkable difference between both 100k resistors, (1) the black, naked one, and (2) the red one in the box. When I measured the red HP pww resistor (2) directly, w/o the case, the error vanished, and also its frequency response was smooth. The capacitance of the case is about 40pF. This gives a time constant on the order of µsec, which cannot affect the reading. When I measured the leakage current of the jacks, I finally found a relaxation effect of the expected order of magnitude and time scale. This current decreases slowly, over several seconds from about 20nA to several hundred pA, where it stabilizes. That happens each time, when reversing the voltage. Therefore, all precision resistor measurements with OCOMP functionality, on values of 1k and above have to be carefully analyzed for parasitic capacitance (order of nF and more), but also for Polarisation effects, coming from the plastic isolation in jacks and cables. This presumably also affects standard resistors like the ESI SR104 or the WEKOMM type. Choosing an appropriate delay of 1 .. 10sec, OCOMP, and 4W connection then may allow measurement uncertainties of < 1ppm. |
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